How are planets born? LkCa 15 could give scientists first-ever peek.

A newly discovered planet-candidate circling star LkCa 15 could provide astronomers with a first-ever look at a gas-giant planet as it grows. It could help prove or debunk current theories.

Karen L. Teramura/UH IfA.
An artist's rendering of what the planet-candidate circling LkCa 15 might look like.

For the first time, astronomers say they have caught a fledgling gas-giant planet in the act of growing.

The researchers are cautious – dubbing it a "likely" protoplanet. Still, if the team's conclusions hold up, the observations would help fill a crucial gap in a pictorial history planetary scientists have been assembling as they strive to unravel the mysteries of planet formation.

The planet appears to be building itself from surrounding dust and gas as it orbits its host star. Such observations provide reality checks on the computer simulations astrophysicists have developed to lay out the processes that form and shape solar systems – from the collapse of enormous interstellar clouds of dust and gas to star formation to the emergence of planets.

Indeed, the observation appears to bolster one of two contending explanations for how gas giants form, other researchers say.

The announcement, released to coincide with a conference this week on solar-system formation at NASA's Goddard Space Flight Center in Greenbelt, Md., comes during a decade-long explosion in the study of planets orbiting other stars and the formation of planetary systems.

Since the mid-1990s, astronomers have detected just under 700 planets orbiting other stars, with hundreds more awaiting confirmation from projects such as NASA's planet-hunting Kepler mission. Researchers also have detected many stars surrounded by dust and gas out of which planets can emerge.

"Catching planets as they grow as been a goal for quite a while," says Adam Kraus, an astronomer at the University of Hawaii who conducted the study, along with Michael Ireland, an astronomer at Macquarie University in Sydney, Australia.

"We know that once planets form, they tend to move around," settling into orbits different than the ones they occupied at birth, Dr. Kraus explains. "You want to capture planets as close to their formation as possible so they are as good indicators of the formation process as possible."

Now, Kraus and his colleague appear to have caught a planet in the act.

The protoplanet's host star is known as LkCa 15, one in a catalog of some 160 stars surrounded by broad disks of debris, perhaps dust and gas, according to published research.

The star lies 457 light-years from Earth in the constellation Taurus. LkCa 15 is only about 2 million years old. The sun, by contrast, is about 4.6 billion years old. LkCa 15 has about the same mass as the sun.

In 2005, scientists using NASA's Spitzer Space Telescope, an orbiting observatory that can spot dust using infrared wavelengths, conducted a survey of stars with dusty disks and noted several that appeared to have gaps in the disks – including LkCa 15.

Three years later, another group published follow-up observations of LkCa 15 and confirmed a pronounced gap in the disk – just the sort of gap that might have been cleared of material by protoplanets feeding off of it.

The observation showed an inner dust ring that extended between 0.12 and 0.15 astronomical units (the distance between earth and the sun is 1 AU) from the star, followed by a more-tenuous ring out to about 5 AU. From there out to 46 AU, Spitzer saw virtually no evidence of tiny dust grains.

When the star's disk reappeared at about 46 AU, roughly the orbit of Pluto, it showed a very pronounced inner edge – a hallmark of planetary bulldozing.

In February, astronomers using Japan's Subaru Telescope atop Hawaii's Mauna Kea released the most detailed images yet of the disk and its gap, but still no planets.

For their part, Kraus and Dr. Ireland used one of the larger, twin Keck telescopes, also on Mauna Kea, to observe the star. They used techniques to freeze as much as possible the twinkling effect that the Earth's atmosphere imparts to stars.

This allowed the two to spot the pinpoint of light they identify as the protoplanet, as well the dusty disk surrounding and feeding it. The protoplanet and its disk orbit in the middle of the gap that stretches from 5 to 46 AU.

The duo calculates that the planet has six times Jupiter's mass. Based on the protoplanet's brightness, color, and the presence of its surrounding tutu of material, the object "has likely been caught in the epoch of assembly."

For Benjamin Bromley, an astrophysicist at the University of Utah who uses computer models to study solar-system formation, Kraus's and Ireland's observation holds the potential to provide a powerful test of two competing ideas for how gas-giant planets form.

The standard explanation is a kind of boot-strap approach where dust grains gather into clumps, the clumps' gravity attracts more clumps, and a rocky protoplanet builds. Once it reaches about 10 times Earth's mass, its gravity is strong enough to draw in surrounding gas. The process can take a million or so years to occur.

A more recent alternative mirrors ideas of star formation. In this case, gravity from the star and the motion of its surrounding disk by themselves introduce instabilities into the disk. These form relatively dense regions whose gravity triggers something of a runaway collapse of the surrounding material into an orbiting orb. It's a quick process and one that tends to leave no residual disk once it's finished, Dr. Bromley explains.

Given the star's 2 million-year age, the protoplanet's behavior "is really what you'd expect right at this moment" under the standard, clump-accretion explanation.

"This is really exciting," he says.

Kraus and Ireland remain a bit cautious, noting that for now, the object is pegged as a "likely" protoplanet. Confirming its planetary nature will take additional work. Kraus says that the next steps are to observe the system again during the next year or two for signs that the pinprick of light and the dust surrounding it are co-orbiting the star.

In addition, the configuration of the gap in LkCa 15's disk implies the possibility of other planets, which new generations of ground-based and space-based telescopes could spot, he says.

Yet Dr. Bromley notes that one can't rule out the single planet as the sole occupant of the void. The rest of the material that should be there could either be tied up in icy objects too cold to observe or in asteroid-like planetesimals that also fall into an observational no-man's-land – for now. Rocky inner planets of the future?

"Just because there isn't much to see doesn't mean there isn't a lot happening," Bromley says.

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